Bit block



May 5, 1935, E.` P. STENGER Erm. l2,039,747

BIT BLOCK t, -7 I INvE-Nrona W( ATTORNEY:

Patented May- 5,' 1936 A UNITED STATES PATENT OFFICE' cinnati, Ohio, assignors to The Cincinnati Mine Machinery Company, Cincinnati, Ohio, a corporation of Ohio Application October 10, 1933, Serial No. 692,998.-

36 Claims. (Cl. 262-33) This invention relates to mining machinery' and is particularly directed to improvements in cutter bits or teeth andholders for mounting the same in coal cutting chains. The present invention also relates to the construction of the chain block or box for a more eiiicient mounting of the bit and holder relative thereto.

'I he mining machinery to which these teeth are applied is of that type'employing an endless chain. 'I'his chain carries the teeth angularly laterally disposed for making a cut through the coal. The angle of mounting of the teeth is varied in transverse planes, whereby the successive teeth make a cut which is suiliciently Wide to permit easy passage of the run of chain and the cutter bar through the cut. The present invention being primarily directed to the construction of the teeth or bits and to the mounting thereof, it is only necessary for the purpose of clearly describing and illustrating the same `that reference be made to one chain block or link of the cutter chain. Each of these tooth carrying blocks or boxes as they may be termed includes asocket .which is transverse to the longitudinal extent of the chain. n

The average cutter tooth or bit is subjected to extremely damaging usage and it is constantly necessary to replace the bits. Accordingly, it is an object of this invention to provide an improved bit holder and a device orV means for fastening the same in a chain block or box, which elements are lconveniently and quicklyI ma nipulated by the operator for inserting a"new bit or reversing the worn bit, end for end, in cases where a reversible bit is used, thereby reducing bit setting time to a minimum. The ability to quickly set the bit is a distinct time saving advantage sincethe average miner is required to set hundreds of bits per day.

Because of the variant character of material through which the bits are drawn for the purpose of making a cut, a great many bits are lost.

This occurs because of chattering or vibration found inthe construction of the holder and its relation to the box, whereby the tendency to lift the holder from its socket or position in the box is materially lessened.

Due to the excessively hard usage to which the bits are subjected, it is of course highly essential that the construction of the holder and the boxes be extremely rugged and durable so that breakage rarely occurs. Therefore, it is a still further object of this invention to provide an improved `1li holder for the bit in which the heavy forces imposed upon the bit in cutting are properly absorbed and counteracted by proper abutment and location of the bit in the holder, and by proper distribution of the mass of the holder. 'Ihis object may include the provision of replaceable contact elements on the box.

The gauge of the bit, or the distance from the lug to the bit point, in some uses, must be varied according to conditions. Accordingly, it is afur- 2o ther object of this invention to provide a bit mounting or support means which can be in terchanged to procure different bit gauge.

In order to accomplish eflicient coal cutting, it is also necessary that the bit have ample clearance or projection beyond the line of the holder and the box. It is accordingly a furtherv object of this invention to provide a bit holder and mounting in the box or block which because of the durability of the holder and the manner in which the forces acting on the bit are absorbed Within the'holder permits ample extension of the cutting point of the bit without overloading the structure of the holder or the box, whereby it does not split or deform, breakage and distortion of the holder being reduced to a minimum. This construction or arrangement results in a good ratio between bit clearance and the length of the bit which is one of the features constantly sought after in this type of 40 apparatus since itjhas a very important bearing, on the cost of bits.

Other objects and further advantages will lbe more fully apparent from a description of the accompanying drawing, in which: A4,5

Figure 1 is a side view of a length of mining machine chain incorporating the improved cutter teeth and mounting therefor.

Figure 2 is a fragmentary end view of a cutter carrying block of the chain. A

Figure 3 is a fragmentary sectional view taken on line 3 3, Figure 2, illustrating, the bit, bit holder, and block construction in detail.

Figure 4 is a perspective view of one of the replaceable abutment plates of the block.

Figure is a perspective view of the holder and bit assembly. -v v Figure 6 is a sectional view taken similar to Figure 3, but illustrating a modified form of abutment plate and holder fastening screw arrangement. A

-'Figure 7 is a perspective view of a modified abutment plate of Figure 6.

' at Il, the links of the chain being held together at the respective sides of the cross groove.

by means of pins I2 in the usual manner.

Each bit carrying link or box has a lug portion I3 extended from one side thereof. These lugs are cross grooved as at I4 and include sockets I5 extending from the bottomv of the groove I4 and into the interior of the link, the transverse angle of the socket being varied as to the successive boxes as desired for the purpose of disposing the teeth at different angles for increasing the width of the cut made by the successively applied teeth.

Each bit or pick holder I6 preferably includes a head I1 and shank I8, the shank insertable into the socket I5 and the head lying in the cross groove I4 of the lug. It will vbe observed that the chain box or block is constructed so that the holder can be faced in either direction longitudi- `nally of the chain, depending on the vdirection of drive of the chain. Therefore the construction of the lug-may be duplicate as to the respective sides thereof.

Duplicate abutment plates I9 may be mounted Each plate includes a stud projecting from one face and has a cross groove 2Ivin its other face including an inclined portion 22 at one end thereof. When the plate is fitted against the side of the groove of the lug the stud 20 fits in a bore 23 A extended through the side wall of the lug formed by the cross groove Il. A reinforcing cross rib or bead 2l is provided at the outer or same side of the plate face as the stud and extends along the upper edge thereof. In ilxed position of the plate, the plate lower edge engages the bottom of the cross groove, thereby preventing rocking of the plate on the stud and insuring a rigid attachment for the plate. I

The holder I6 includes an obliquely disposed opening 25, diamond shaped in the present case for receiving the bit which is cut in this instance from bars of diamond shape cross section. This `places the blt in oblique position or forwardly inclined position when the holder is inserted in the lug. The lbit hasboth ends sharpened by obliquely cutting the ends of the diamond shaped length of cutter bit material, the end surfaces 2B appearing from the same side of the bit. The lower portion or heel oi' the outwardly extended one of these obliquely cut ends is seated in the angular portion 22 of the plate groove 2|. The end face 26 of the-,bit inserted in the holder rests against a cross pin 21 of the holder and the inner end of the bit is held clear of and cannot be damaged by contact with the opposing abutment plate, the pin Bunting insertive movement of the bit.

The bits may be described as trapezoidal in side elevation. One of the'parallel sides of each bit is considerably shorter than either of the respectivel oblique end faces thereof. 'I'hat is to say. the trapezoid has two parallel sides. of unequal length. It is preferable that the short side of the bit be of a length not less than one-half the transverse thickness of the bit, that is. the thickness on the minor axis, and not more than onevhalf the length of eitherof therespective end faces or surfaces.

" Due to the engagement of the bit with the abutment block the bit holder is maintained in a definite position of insertion in the socket. This causes the head of the holder to be cradled in the groove, the shoulder 28 between the head and ioY yshank preferably being clear of the bottom of the f lug groove. The screw 29 for securing the holder in position in the block is threaded through the bore at a particular side of the lug portion of the block and has a flat dog pointy which engages4 A lug or lip 3llEL is formed at the lower end thereof.

It will be observedA from the foregoing detailed description that-blt setting time is reduced to a minimum in the present construction because removal of the bit is accomplished by thesimple act of backing oif the set screw a fraction of a turn and then` moving the holder outwardly in the socket, the outward movement of the holder being, limited by the lip 30, after which the bit is free to be withdrawn from the holder for the purpose of reversing cutting ends or insertion of a`new bit without entirely withdrawing the holder from the lug. Accordingly, reinsertion of the holder in the darkness of the mine is unnecessary which is an important step in eiliciency. No time is lost in adjusting the various clamping members as is the case when other than a one-piece holder is used. When it is remembered that a miner is often required to set hundreds of bits a day, it will be realized that savings in bit setting time can Vappreciably increase the amount of coal cut perl i ing consists of a number of parts clamped together. Such parts seldom seat or align themselves perfectly and are often jarred loose and lost during the operation of making the cut. In

the present construction the loss of bits and hold- Y ers in service is practically eliminated.

A different type of abutment plate may be utilized (see Figure 7). In this type a small lug 3| is projected from the outer face of the plate and engages in a notch 32 in the lug, the inner face of the plate being grooved on an angle as at 33 for receiving the lower portion of the oblique end of the bit.

It will be seen that the inner end of thc bit rests on the pin 21 which absorbs the thrust of the bit resulting from the forces encountered in the cutting operation. Inasmuch as the bit thrust is absorbed within the head of the holder itself. there is no wedglng action tending to'lift or force the holder out of the socket. Instead of the bit holder resting on top of the block, it is merely contained in the cross groove with contact at the rear with the adjacent side of the groove ofthe lug. The holder cannot be forced backwardly because of the support it receives by abutment against the removable abutment plate I9 of the' lug.

The engagement of the bit with the abutment plate at an acute angle forms a stop 34 which prevents the bit from moving outwardy or being removed unless the holder is lifted up a slight distance 4out of the cross groove. 'Ihe groove inthe block not only provides for extremely rigid and secure abutment for the holder and bit but also provides ample space so that the head of the holder can be made more sturdy than would otherwise be the case.

For e'icient cutting, itis necessary that the bit have considerable clearance, that is to say, the bit should project a suiicient distance beyond the holder and its mounting for efdcient cutting results. However, unless the holder is very sturdy or the bit uneconomically long, the leverage of the projecting bit will cause the forces acting on it to split or deform the holder. As stated above, the recess or cross groove in the block permits the head of the holder to be made of considerable cross section or mass and amply strong and rigid to keep breakage and distortion of the holder to a minimum. AAs a result of this the life of the holder is unusually long and this combined with the low loss of bits and holders and quick bit setting time makes for economical operation and low cutting costs.

As mentioned above, sufdcient bit clearance isA readily obtained in this construction with a rela.- tively short bit. This is made possible by the shape of the bit and the manner in which theA bit is supported on its inner obliquely cut surface on the cross pin. The bit is so disposed toward the pin `and the bit receiving passage that the stresses resulting from the thrust of the bit are not of a localized character and consequently are not as -harmful as would otherwise be the case.

Some mining practices necessitate a variable bit gauge, that is to say, it is necessary to varyv the distance from the point of the bit or pick to the box.4 In what is known as long wall mining,

the same cut is continued in a long stretch of coal-face. Often in this type of mining the chain carrying bar or jib becomes tilted up or down by the varying nature of the substance being cut. By changing the gauge of certain of the bits, this tilting may be counteracted, since a heavier` cut element in which the seat is higher may be inserted, thereby setting `the holder and bit out in the socket'.

, A higher bi1; gauge can rie-obtained with the same abutment plate by placing shims between' at various points where abutment is made, such as on the stop pin in theaholder, the rear of the j holder head and various other places.

As described heretofore and particularly disclosed in Figure 3, the holder is reversible. This reversibility is provided for the purpose of operating the chain in long wall mining. In long wall mining the cut is made back and forth along the same long face of coal. This necessitates reversing the direction of chain travel after each cut is made. Accordingly, the present bit mounting is quite adaptable for long wall mining.

If desirable the set screw can be permanently located on one side of the lug and the holder shank can be properly faced on both sides to be engagedby the set screw, (as shown on one side in the drawing), for set screw contact on either side. By the provision of duplicate opposite side faces on the shank the holder is readily reversed and eiiciently held without removing and transhferring the set screw from one side of the lug to the other.

Referring to Figure 3 or 6 of the drawing, the following advantages of rigidity and secure attachment of the holder are present regardless of the direction in which the bit is directed. The

abutment'between the plate I9 and the adjacent side of the head ofthe holder occurs at an equal distance from the point of the bit regardless of whether the set screw is engaging one or the other of the opposite faces of the holder because the engagement or contact of the end face of the holder is the same for either direction. When the holder and bit are placed in the chain lug, the llower portion of the forward end surface of the bit rests against the abutment plate and the rear surface of the head of the holder rests against the chain lug. As the set-screw is moved against the side-surface of the shank of the holder, the

holder is rotated slightly as the holder shank moves in an arc in the normal clearance of its socket under the inuence of the set-screw. This rotating action may be described as a rocking or cam action and takes place on the contacting surfaces of the holder head and the lug. This rotation of the holder rigidly locks the bit in posi-l tion by increasing its pressure against the abutment plate. tageous, for with abutment always insured between the rear side of the holderhead and the adjacent abutment plate, the stresses imposed on the holder will not be excessive as would be the This construction-is highly advancase when the rear abutment occurs at the point where the set screw makes engagement. The long leverage in the latter instance would causelthev holder to becomebent or broken.

Having described our invention, we claim:

1. In a mining'machine chain, a link including a lug portion, a holder socketed in said lug, said holder including a head portion and said lug including a holder-head receiving recess, an -abutment plate removably mounted in said lug adjacent the forward side of said head,A a set screw for holding said holder in place, and a cutter bit mounted in the head and projecting angularly therefrom'beyond the lug, said bit overlying said removable abutment plate and limiting insertive movement of the holder.

2. A bit carrying box for a gnining machine chain including a recess and a socket extended inwardly from said recess,.a holder including a; head and a shank, the shank engageable in. the socket and the head disposed insaid recess, said head including a cutter bit receiving opening, a

' cutter 'uit mounted in said opening and extended angularly forwardly from the head, and an'abutment plate secured/against the forward wall of said recess, said cutter bit having an end engaging said abutment plate, said abutment plate hav- X ing a stud extended fromits forward face into a shank, said box including a recess for contain-V ing said head, the head of said holderhaving an obliquely disposed blt receiving opening therein,

a cross pin disposed through said head across said opening, a reversible bit insertable in said vopening having oblique end faces appearing from the same side ofl the bit, said box including a seating surface adjacent the forward side of the holder-head for seating the outer oblique end face of the inserted cutter bit, the inner end face resting upon said cross pin. Y

4. A mounting for a coal cutter bit, comprising, a chain lug including a socket, a holder including a head and a shank,l said lug including a recess for containing and supporting said head, the head of said holder having an obliquely disposed bit receiving opening therein, a reversible bit insertable in said opening having oblique end faces facing toward the same side of the bit, said lug including a seat in the side of its recess adjacent the forward side of the holder-head for seating the outer oblique end face of the inserted cutter bit.

5. In a mining machine' chain, a link including a lug portion, a holder socketed in said lug, said holder including a head portion and said lug including a head receiving portion; abutment plates mounted in said lug at the front and rear sides of said head, Aa set screw for holding said holder in place, and a cutter bit mounted in the head andvprojecting angularly therefrom, said blt seatcontact with .the rear wall of said recess, means for holding said holder in place, and a cuttefr'bit obliquely mounted in the head and projecting l, angularly therefrom. A

7. .Al bit carrying lug for a mining machine "chain including a socket anda cross groove, a

holder including a head and a shank, the shank engageable in the socket and the head disposed' engaging the shank of theholder angularly to- A ward the inner end of the socket.

8. -Amounting for a coal cutting bit, comprising, a chain l'nk, -a holder socketed in said link, said holder including a head portion and said link including a grooved lug portion for containing said head, an `abutment plate mounted in said lug at the front side of said groove, means for holding said holder in place, and a cutter bit mounted in the head portion and projecting angularly therefrom beyond'the lug, said bit bearing against said abutment plate, whereby said bit is held against removal unless the holder is moved slightly outwardly in vits socket.

9.a bit mounting. comprising. a block, a] bit, a bit holder mounted in the'block including an obliquely disposed bit receiving and supporting socket in its outer end, said socketin'cluding a support -for the inner end vof said bit, means for `securing Vthe bit holder'in the block, and a relmovable bit seating plate mounted in the block of the bit apart from the support aorded by the holder.

10. A bit mounting apparatus, comprising, a

block including a shouldered socket, a bit holder including a head and a shank,said shank insertable in said socket, the rear of said head engaging the rear side of the socket, said head including an obliquely disposed bit socket, a bit mounted in said socket projecting forwardly and sertable in said sockethaving an oblique end face atl its cutting end, said block including a. seat in the side of its recess adjacent the forward side of the holder-head forming a rest and abutment for the oblique end face of the bit.

12. In a device of the class described, a bit carrying box including a socket, a holder engageable in said socket, said holder including an obliquely cutter bit receiving opening having a support for the inner end of the bit therein, a cutter bit mounted in said opening against said support and extended angularly forwardly from the holder, said cutter bit having `a portion of its forward end face engagingthe wall of said socket,

and means forA securing said holder in place.

13. A mounting for a bit/comprising, la box including a socket, aholder mounted in said socket,

said holder having an obliquely disposed bit recelving opening therein, a reversible bit insetable.

in said opening havin oblique end faces appearing from the same de of the bit, said. holder including an abutment disposed in said opening supporting the inneral end of said bit, said box including a seating surface adjacent the forward sde of the holder for seating the outer oblique end face of the inserted cutter bit.

14. In a mining machine chain, a chainlink including a lug portion, said lug portion including a socket, a holder mounted in said socket having a b't socket therein including a support'for the inner end of the bit, a cutter bit mounted in the socket ofthe holder against said support and projecting angularly forwardly therefrom, and a clamping device laterally engaging the portion of the holder mounted in the socket of the `lug for l from, a clamping device laterally engagingthe portion of the holder mounted in the socket of the lu'g for securing said holder rigidly in pos'ition in its socket for binding the bit in its socket; by the engagement of the bit with the lug, and an abutment for preventing outward movement offsaid bit.

l 16. In a device of the class described, a block vincluding a socket, a blt holder engaged in said socket and having a bit receiving socket therein a bit mounted in said bitreceiving socket against said support thereof and .projecting angularly engages the outer end'y of the bit against the block.

17. A bit carrying block for mining machine chain, comprising, ablock including a socket, a bit holder secured in said socket including a bit receiving socket, a bit mounted inthe bit receiving socket of said holder andv projecting angularly forwardly from the holder, said bit extended within the socket of the blockin position of use and having an end face engaging the block, the

inner vend of said holder` including a lip, a set screw in the block engaging the side of the holder above the lip and adapted to be engaged by the lip as the holder is withdrawn, said lip and set screw spaced apart a distance sufilcient for per mitting outward withdrawal movement of the holder for clearing the bit from engagement with the block.

18. In a mining machine chain, a lug including a socket, a holder mounted in the socket of said lug having a bit receiving socket therein, a bit removably mounted in the bit receiving socket of said holder and projecting angularly forwardly from the extended end thereof, and a set screw in said lug adapted to engage the adjacent side of the holder, said bit having an oblique end face engaging the lug, the extreme upper portion of the rear side of the holder head engaging the lug, said set screw inclined downwardly inwardly in the lug for moving the holder in an arc in the socket and causing more positive engagement of the oblique end face of the bit with the lug.

19. A bit carrying lug for a mining machine chain, including a socket, aholder mounted in i said socket, said holder including a cutter bit receiving opening including a support for theinner end of the cutter bit, a cutter bit mounted in said opening against said support and extended angularly forwardly from the holder, and an abutment plate secured against the forward wall of said socket, said cutter bit having its extended outer end in engagement with said abutment plate. f

20. In a mining machine chain, a link including a lug portion, said lug portion including a socket, a holder mounted in said socket including a bit 'receiving socket having a support for the inner port therein and projecting angularly therefrom beyond the lug, said bit engaged against said removable abutment plate, and means for securing said abutment plate in position, whereby said 'abutment platefis maintained against displacement when saidholder is in position in the socket of said lug.

' 2l. A bit carrying lug for a mining machine chain, including a socket, a holder mounted in abutment plate, said abutment plate having a stud *extended from its forward face into said lug, and y means on said lug engaged by said abutment plate for preventing rotation thereof on said stud.

22.v In a mining machine chain, a lug including a socket, a holder mounted in the socket of said 5 lug, a bit removably mounted in said holder projecting angularly forwardly from the extended end thereof, and a set screw in said lug adapted to engage the adjacent side of the holder, said bit having an oblique end face engaging the lug, -10

the extreme upper portion of the rear side of the holder head engaging the lug, said' set screw maintaining the holder head and the bit in contact with'the lug.

23. In a mining machine chain, a lug including l5 a socket, a holder mounted on said lug and including a shank disposed in said socket, a bit having its inner end supported inA said holder and projecting angularly forwardly in contact with the lug, and a set. screw in said lug inclined down- 20 wardly inwardly in the lug for forcing the shank of said bit laterally in said socket.

24. A mounting for a bit, comprising. a box including a socket, a holder mounted in said socket. said holder having an obliquely disposed 25 bit receiving opening therein, a reversible bit insertable in said opening having oblique end faces appearing from the same side of the bit, said holder including an abutment disposed in said opening supporting the inner end of said bit, said 30 clamping thebit between .the holder and the seat- 35 ing surface.

25. In combination with a bit holding lug `of a. mining machine chain, a bit holder engaging within a socket in the chain lug, a bit having oneend thereof socketed and seated within the head 40 of said holder and an opposite cutterend protruding therefrom, the protruding end of the bit surfaced for engaging with the lug compressing the bit within the holder socket and clamping the same between the holder and lug upon urging 45 the holder into the lug socket, and screw means cooperating with the holder for securing the same f in the lug socket.

26. A bit carrying device for use in a cutter chain, comprising, a box having a socket therein, 50

a holder element mounted in said socket, said element including an obliquely disposed cutter bit receiving bore therein, a cutter bit mounted in said .opening in position with a cutting end thereof extended angularly forwardly beyond the 55 holder element, said cutter bit having the aforesaid cutting end resting on the box at the forward side wall of lthe socket in the box, said holder element having the extreme upper portion of its rear surface engaging therear surface of the 00 socket in the box, and means for securing said holder in position.` V

27. A mounting for a cutter bit, comprising, a block having a socket therein, a bit holder mounted in said socket, 'said holder including a 65 forwardly inclined bit receiving passage, a cutter bit mounted in said passage` and extending angular-ly forwardly from the holder, said' cutter bit having a portion of its forward end engaging the block at the forward side wall of said socket in the block, means within the holder for limiting'insertive movement in said-bit, and means for securingsaid h'older in position.

'28. In a cutter chain, a bit block having a socket,

a holder mounted in said socket including an angularly disposed socket therein, a cutter bit having obliquely extended end faces providing cutting points, said cutter bit mounted in said socket of said holder, said block including a portion ex`- tended outwardly, and a removable abutment plate in said portion of the block and having an inclined surface engaged by the outer end face of the bit opposite to the point thereof, said contact of the abutment plate and cutter bit pro- Viding the only contact -of from the holder. Y

, 29. In a cutter chain, a bit block having a' lug portion, said lug portion having a socket therein, and extensions projected upwardly from the lug at the front and rear ofvsaid socket. 30. In a cutter chain, a bit block having a. lug portion, said lug portion having a socket therein, extensions projected upwardly from the lug at the front and rear of said socket, and an abutment plate Secured in one of said extensions against its inner face.

31. A reversible coal cutting bit formed from bar stock having cutting ends presenting fiat opposingly obliquely disposed surfaces providing cutthe cutter bit apart 'ting points aty the same side of the bit, said bit of trapezoidal form in side elevation, the short side of which is of a length not less than one-'half the transverse thickness of the bar stock and not more than one-half the length of either of the respective end surfaces.

32. A reversible coal cutting tooth diamond shaped in cross-sectionhaving obliquely disposed end surfaces appearing from the same side of the tooth and providing cutting points located at the same longitudinal side edge of the tooth, said tooth of trapezoidal form in side elevation, the short side thereof being of a' length not less than one-half the transverse thickness of the tooth and not more than one-half the length of either of the respective obliquely disposed end surfaces thereof.

33. In a cutter chain, a chain lug including a socket, a bit holder mounted in said socket, said bit holder having an langularly disposed socket against the lug.

therein, a bit mounted in said socket of said holder, said bit of trapezoidal outline in side elevation and having a short side of a length not less than one-half the transverse thickness of the bit and not more than one-half the length of either of its respective end surfaces, the outer end surface of the bit resting on the chain lug, and means for securing the holder in the chain lug.

34. A cutter bit adapted to be mounted in a coal cutter chain, the cross section of said bit having a major and minor axis and symmetrical on these axes, said bit of trapezoidal outline in side elevation, one parallel side of said trapezoid being of a length not less than one-half the thickness of the bit on its minor axis and not more than onehalf the length of either of the remainingsides, the obliquely disposed end surfaces forming a cutting point at each end of the bit at the saine side thereof. v

35. In a cutter chain, a chain lug, a bit holder having a head and a shank, said shank socketed in said chain lug, and said head having its rear surface engaged against the chain lug, a bit angularly mounted in the holder head and projecting forwardly therefrom with its active cutting end surface engaging the lug, and means engaging the shank of the holder causing the holder to rock on the lug on the rear contacting surface thereof for clamping the forward end 36. A reversible cutter bit formed from diamond shaped bar stock, providing obliquely disposedsurface of th.` bit Y site to said first mentioned longitudinal side edge,

said latter edge being of a length not less than one-half the thickness of the bit on its minor axis and not more than one-half the length of -either of the respective end surfaces of the bit.

EDWIN P. S'I'EDTGER.

ARMIN O. BRUESTLE. 

